1. Field of the Invention
The present invention relates to a circuit for driving a liquid drop spraying apparatus used for a variety of machines, the liquid drop spraying apparatus treating a liquid or operating by spraying the liquid as a small amount of liquid drops.
2. Description of Prior Art
A liquid drop spraying apparatus is generally composed as shown in a longitudinal cross section of FIG. 7, including a piezoelectric or electrostrictive element 1 (piezoelectric/electrostrictive element) as a pressurizing means for spraying liquid. The piezoelectric/electrostrictive element 1 is provided on a wall face of a pressurizing chamber 2 for pressurizing liquid to be sprayed, a liquid drop spraying nozzle 3 is provided at the tip end of the pressurizing chamber, and an introducing hole 5 for supplying liquid to the pressurizing chamber 2 is formed at its proximal end, thereby entirely constituting a liquid drop spraying unit 6. This liquid drop spraying unit 6 is concurrently formed in plurality, and an introducing hole 5 for a plurality of the adjacent liquid drop spraying units 6, 6, . . . is coupled with a common liquid supply path 7.
A drive circuit for driving a liquid drop spraying apparatus deforms the wall of the pressurizing chamber 2 by applying a predetermined voltage signal to the piezoelectric/electrostrictive element 1 and charging the element. In this manner, the drive circuit generates a pressure at the pressurizing chamber 2, and causes the liquid supplied to the pressurizing chamber from the liquid drop spraying nozzle 3. In addition, the drive circuit causes power discharge and release deformation, thereby restoring the deformation of the pressurizing chamber 2, and causes liquid to flow from the introducing hole 5 to the pressurizing chamber.
In the meantime, in the liquid drop spraying apparatus, a large amount of liquid must be supplied for certain uses. In order to cope with such uses, the apertures of the nozzle and introducing hole have been increased in size.
If the aperture of the liquid drop spraying nozzle 3 is too large, however, only a small amount of liquid can be sprayed. The introducing hole 5 is not a mere path through which liquid is supplied to the pressurizing chamber 2, but serves to prevent back flow even if pressurization is performed so as to spray a small amount of liquid drops from the liquid spraying nozzle 3. Thus, the aperture cannot be widened infinitely.
Therefore, it is considered that if the time interval for applying a predetermined voltage signal to the piezoelectric/electrostrictive element 1 is shortened, the number of signal applications per unit time is increased, and the number of supplies to the liquid drop spraying apparatus is increased. In that case, if the time interval for applying the voltage is shortened, there occurs a new problem, in that a delay in liquid supply from the introducing hole 5 to the pressurizing chamber 2 occurs, and a large amount of liquid cannot be constantly supplied.
In addition, the piezoelectric/electrostrictive element acts as a capacitor. Charging/discharging is repeated when the spraying operation is repeated. Thus, when an operating period is shortened, power consumption is further increased, and the calorific value is increased.
A technique to cope with the above increased power consumption is described in Japanese Patent Application Laid-open No. 10-107335 or Japanese Patent No. 2909150. A technique for shortening the application period of a voltage signal per unit time is disclosed in Japanese Patent Application Laid-open No. 8-300646. In Japanese Patent Application Laid-open No. 10-107335, there is disclosed an arrangement in which an external capacitor is provided for power recollection, a coil that is an inductance is interposed in a charge/discharge circuit, and part of a discharge of the piezoelectric/electrostrictive element is effectively stored in the external capacitor and utilized for the next charge, thereby ensuring power saving. In Japanese Patent No. 2909150, there is disclosed an arrangement in which piezoelectric elements to be driven at different timings are mutually utilized as power recollecting means, thereby reducing power consumption without additionally providing an external circuit.
However, in all of these arrangements, although there is provided an advantageous effect on saving power consumption, a technique for constantly spraying a large amount of liquid is not described. Thus, the delay in liquid supply from the introducing hole to the pressurizing chamber is not solved.
In addition, in Japanese Patent Application Laid-open No. 8-300646, there is disclosed an arrangement in which the rise of a drive voltage wavelength is divided into two stages and/or three stages, whereby the stability of meniscus is improved and the occurrence of satellite drops is suppressed. As a result of reducing a constant during discharge when the waveform is divided into three stages, the printing speed is increased, thereby making it possible to increase an ink discharge quantity. However, discharge start characteristics are not smooth at the respective stages. Thus, the printing speed cannot be significantly increased, and the discharge quantity cannot be significantly increased.
In view of the foregoing problems, even in the case where a large amount of liquid is sprayed, it is an object of the present invention to provide a circuit for driving a liquid drop spraying apparatus capable of smoothly supplying liquid to the pressurizing chamber.
In order to solve the foregoing problems, according to a first aspect of the present invention, the following is provided. A liquid drop spraying apparatus comprises a plurality of units for spraying a small amount of liquid drops, wherein each unit comprises a liquid drop spraying nozzle, a pressurizing chamber for pressurizing liquid to be sprayed from the nozzle, an introducing hole for supplying liquid to the pressurizing chamber, and a piezoelectric/electrostrictive element for operating the pressurizing chamber to be pressurized. The liquid introducing holes of the plurality of the adjacent liquid drop spraying units are coupled with a common liquid supply path, and there is provided a liquid drop spraying apparatus driving circuit for applying a predetermined voltage signal to the piezoelectric/electrostrictive element, thereby deforming the wall of the pressurizing chamber, and discharging from the nozzle the liquid to be supplied to the pressurizing chamber by a pressure produced in the pressurizing chamber. The ratio between the introducing hole diameter and the nozzle hole diameter (introducing hole diameter/nozzle hole diameter) is in a range of 0.6 to 1.6, and the ratio between the nozzle hole diameter and nozzle thickness (nozzle hole diameter/nozzle thickness) is in a range of 0.2 to 4. An applied voltage signal supplies a current to the piezoelectric/electrostrictive element to charge the element, and then holds the final charge voltage for a predetermined time after applying the current. Discharging with two or more time constants during discharge is then performed sequentially. The time constant during the first discharging is greater than the time constant during the second discharging, and the second discharging is started at a voltage that is 35% to 70% of a voltage difference between the charging start voltage and the final charge voltage when the charging start voltage is defined as a reference. An inductance and a resistor are interposed in at least one discharge circuit in series with respect to the piezoelectric/electrostrictive element.
With this arrangement, when liquid drops are sprayed during the charging of the piezoelectric/electrostrictive element, when a charge is discharged, the time constant during discharging of the first discharge circuit is large, and thus, the piezoelectric/electrostrictive element can start deformation gently. In the case where liquid drops are sprayed simultaneously from a plurality of liquid drop spraying units, an operation for introducing liquid into a plurality of pressure chambers is reliably performed. Then, this operation goes to a suction operation in which the time required for discharging a unit voltage value is short. Thus, liquid is supplied to the pressurizing chamber smoothly and within a short time, and the amount of liquid supply can be increased.
If the second discharging begins at a voltage less than 35%, gentle suctioning (at the first discharge time constant) dominates too much of the entire suctioning step. Suctioning itself is reliably performed, but a large amount of suctioning per unit time cannot be provided. As a result, the spraying period cannot be shortened, and a large amount of spray cannot be ensured. In addition, if the time constant of the first discharging is comparatively small in a range which is greater than the time constant during the second discharging so as to provide the suction quantity per unit time, unstable suctioning starts, resulting in a spray fault. In addition, if the second discharging begins at a voltage more than 70%, discharging at the larger, first discharge time constant (i.e., gentle suctioning) is too small to smoothly start liquid suctioning. Then, the suction quantity of liquid from the liquid introducing hole to the liquid pressurizing chamber after discharge is decreased, and the entrapment of air bubbles from the liquid discharge nozzle occurs, resulting in unstable spraying.
Further, when spraying is performed as described above, a large ratio between the nozzle and the introducing hole (introducing hole diameter/nozzle hole diameter) is preferable in consideration of suctioning. However, the rate at which the pressure during spraying escapes to the introducing hole side is large, and the spraying force is shortened. Alternatively, if the above rate is small, it causes shortage of the quantity of liquid supply against the spray quantity. Thus, the ratio between the introducing hole diameter and the nozzle hole diameter (introducing hole diameter/nozzle hole diameter) is preferably 0.6 to 1.6.
Furthermore, the ratio between the nozzle hole diameter and nozzle thickness (nozzle hole diameter/nozzle thickness) is preferably 0.2 or more and 4 or less. When the ratio is 4 or less, the residual vibration of the liquid level immediately after spraying can be converged speedily by a contact resistance between the liquid on the nozzle wall face. Still furthermore, air bubbles can be prevented from entry into the pressurizing chamber with the pressure change in the pressurizing chamber during discharge, and the spray stability can be improved. As a result, spraying can be done in a short period of time, and the spray quantity can be increased. In addition, when the ratio is 0.2 or more, there can be prevented a spray fault generated due to the shortage of the spraying force with a large contact resistance of the liquid on the nozzle wall face.
Yet furthermore, when the ratio between the introducing hole and the nozzle hole, the ratio between the nozzle hole and the nozzle thickness, and the discharge voltage rate are met simultaneously, the spray fault due to air bubble entry is prevented, whereby a large amount of spray can be ensured.
According to a second aspect of the present invention, as in the first aspect, there is provided a circuit for driving a liquid drop spraying apparatus, wherein a time (t4), measured from the time when discharging is started at the second discharge time constant to the time when the next predetermined voltage signal is applied to the piezoelectric/electrostrictive element, is xc2xc or more and 20 times or less than that of a specific vibration period (To) when liquid is supplied to a flow path composed of the liquid spraying nozzle, the pressurizing chamber for pressurizing the liquid discharged from the nozzle, the introducing hole for supplying liquid to the pressurizing chamber, and the piezoelectric/electrostrictive element for operating the pressurizing chamber to be pressurized. The ratio (t3/t4) between a time (t3) when discharging begins using the first discharge time constant and the time (t4) is in a range of 0.1 to 20.
With this arrangement, when the time (t4), from the time when the piezoelectric/electrostrictive element starts discharging at the second discharge time constant to the time when the next predetermined voltage signal is applied, is less than xc2xc of the specific vibration period (To), the suction speed of liquid from the liquid introducing hole to the liquid pressurizing chamber after spraying is too fast. Even if suctioning is started at the first charging without any fault, the liquid supply from the introducing hole is too late when suctioning is effected during the second discharging. Thus, air bubbles enter the pressurizing chamber from the liquid drop spraying nozzle, resulting in a spray fault. In addition, when the rate is more than 20 times To, a large amount of suction per unit time cannot be provided. As a result, the spray period cannot be shortened, and a large amount of spray cannot be ensured.
Further, when the ratio between a time (t3), when discharging is effected at the first discharge time constant, to a time (t4), when a predetermined voltage signal is applied to the next piezoelectric/electrostrictive element, is less than 0.1, the rate of the first discharging with its large time constant is small. Thus, the ratio of the liquid suction quantity during the first discharging to the entire suction quantity decreases, and suctioning is too late during the second discharge suctioning. Then, air bubbles enter the pressurizing chamber from the liquid drop spraying nozzle, causing a spray fault. In addition, when the above ratio is more than 20, a large amount of suction quantity per unit time cannot be provided. As a result, the spray period cannot be shortened, and a large amount of spray cannot be ensured.
According to a third aspect of the present invention, as in the first and second aspects thereof, in addition to a discharge circuit, an inductance and a resistor are interposed in series with respect to the charge circuit.
With this arrangement, the voltage/time gradient during spraying is made linear, and the stability of liquid spraying is improved.
According to a fourth aspect of the present invention, in a liquid drop spraying apparatus comprising a liquid drop spraying nozzle; a pressure chamber for pressurizing liquid discharged from the nozzle, and a piezoelectric/electrostrictive element for operating the pressure chamber to be pressurized, wherein the liquid introducing holes of a plurality of the adjacent liquid drop spraying units are coupled with a common liquid supply path, there is provided a discharge circuit for a liquid drop spraying apparatus for repeatedly discharging a piezoelectric/electrostrictive element to which a predetermined voltage signal has been applied, thereby changing the wall of the pressurizing chamber and discharging from the nozzle the liquid supplied to the pressurizing chamber with a pressure generated in the pressurizing chamber. The ratio between the introducing hole diameter and the nozzle hole diameter (introducing hole diameter/nozzle hole diameter) is in a range of 0.6 to 1.6, and the ratio between the nozzle hole diameter and nozzle thickness (introducing hole diameter/nozzle thickness) is in a range of 0.2 to 4. The discharge circuit discharges a current from the piezoelectric/electrostrictive element to which a charge voltage has been applied, and then holds the final discharge voltage for a predetermined time. Charging with two or more different charge time constants is then performed sequentially, and the second charging is started at a voltage that is 30% to 65% of a voltage difference between the final discharge voltage and the discharge start voltage when the final discharge voltage is defined as a reference. The first charge time constant is greater than the second charge time constant. An inductance and a resistor are interposed in series with respect to the piezoelectric/electrostrictive element in at least one charge circuit.
With this arrangement, in the case where liquid drops are sprayed during discharge of the piezoelectric/electrostrictive element, a time constant is large when first charging the charge circuit. Thus, the piezoelectric/electrostrictive element can start shape restoration gently. In the case where liquid drops are sprayed from a plurality of liquid drop spraying units simultaneously, the operation for introducing liquid to a plurality of pressure chambers is reliably performed. Then, this operation goes to a suction operation in which the time required to charge a unit voltage value is short. Thus, the liquid supply to the pressure chambers can be performed smoothly and within a short time, and the liquid supply quantity can be increased.
If the second charging begins at a voltage more than 65%, gentle suctioning (at the first charge time constant) dominates too much of the entire suctioning step. Although suctioning is reliably performed, a large amount of suctioning per unit time cannot be provided. As a result, the spraying period cannot be shortened. Therefore, a large amount of spray cannot be ensured. If the first charge time constant is comparatively smaller within the range that is greater than the second charge time constant so as to provide the suction quantity per unit time, unstable suctioning is started, resulting in a spray fault. In addition, if the second charging begins at a voltage less than 30%, the rate of charging at the first charge time constant (i.e., gentle suctioning) is too small to perform liquid suctioning start speedily, and unstable spraying occurs.
Further, when spraying is performed as described above, a large ratio between the introducing hole diameter and the nozzle hole diameter (introducing hole diameter/nozzle hole diameter) is preferable in view of suctioning. However, the rate at which the spraying pressure escapes to the introducing hole side is large, and the spraying force becomes short. Alternatively, if the ratio is small, it causes a shortage of the quantity of liquid supply to the spray quantity. Thus, the ratio between the introducing hole diameter and the nozzle hole diameter (introducing hole diameter/nozzle hole diameter) is preferably 0.6 to 1.6.
Further, when the ratio between the introducing hole diameter and the nozzle hole diameter, the ratio between the nozzle hole diameter and nozzle thickness, and the charge voltage ratio are met simultaneously, spray faults due to air bubble entry are prevented, and a large amount of spray can be ensured.
According to a fifth aspect of the present invention, as in the fourth aspect thereof, a time (t40), from the time when charging is started at the second charge time constant to a time when the next predetermined voltage signal is applied to the piezoelectric/electrostrictive element, is xc2xc or more and 20 times or less of a specific vibration period (To) when liquid is supplied to a flow path composed of the liquid spraying nozzle, the pressurizing chamber for pressurizing liquid discharged from the nozzle, the introducing hole for supplying liquid to the pressurizing chamber, and the piezoelectric/electrostrictive element for operating the pressurizing chamber to be pressurized. The ratio (t30/t40) between a time (t30), when charging is performed at the first charge time constant, and the time (t40), is 0.1 or more and 20 or less.
With this arrangement, when t40 is less than xc2xc of (To), the suctioning speed is too high. Thus, even if suctioning is started at the first charging without any failure, liquid supply from the introducing hole is too late when suctioning is performed at the second charging. Then, air bubbles enter the pressurizing chamber from the nozzle hole, and spraying cannot be performed. In addition, when t40 is more than 20 times (To), a large amount of suction per unit time cannot be provided. As a result, the spraying time cannot be shortened, and a large amount of spray cannot be ensured.
According to a sixth aspect of the present invention, as in the fourth and fifth aspects thereof, in addition to a charge circuit, an inductance and a resistor are interposed in series with respect to the discharge circuit.
By doing this, the voltage/time gradient during spraying becomes linear, and the stability of liquid drop spraying is improved.
According to a seventh aspect of the present invention, in a liquid drop spraying apparatus comprising a liquid drop spraying nozzle, a pressurizing chamber for pressurizing liquid sprayed from the nozzle, an introducing hole for supplying liquid to the pressurizing chamber, and a piezoelectric/electrostrictive element for operating the pressurizing chamber to be pressurized, wherein the liquid introducing holes of a plurality of the adjacent liquid drop spraying units are coupled with a common liquid supply path, there is provided a circuit for driving a liquid drop spraying apparatus for applying a predetermined voltage signal to the piezoelectric/electrostrictive element, thereby deforming the wall of the pressurizing chamber, and discharging from the nozzle the liquid supplied to the pressurizing chamber, wherein the applied voltage signal supplies a current to the piezoelectric/electrostrictive element, and then holds a final charge voltage for a predetermined time. Discharging with two or more different discharge time constants is then performed sequentially, and the first discharge time constant is greater than the second discharge time constant. An inductance and a resistor are interposed in series with respect to the piezoelectric/electrostrictive element in at least one discharge circuit, the piezoelectric/electrostrictive elements are divided into at least two groups, a circuit for charging and discharging a current is provided at their respective groups, and at least part of the discharge current of one group is used for part of the charge current of the other group.
With this arrangement, when liquid drops are sprayed during piezoelectric/electrostrictive element charging, when a charge is discharged, the discharge time constant of the first discharge circuit is large. Thus, the piezoelectric/electrostrictive element can start deformation gently. In the case where liquid drops are sprayed from a plurality of liquid drop spraying units simultaneously, an operation for introducing liquid to a plurality of pressure chambers can be reliably performed. Then, this operation goes to a suction operation in which a time required to discharge a unit voltage value is short. Thus, liquid can be supplied to the pressure chamber smoothly and within a short time, and the liquid supply quantity can be increased.
In addition, the discharge power of one piezoelectric/electrostrictive element is directly employed as a charge current for the other piezoelectric/electrostrictive element. Thus, there is no need to newly provide discharging means, and further, power consumption can be saved.
According to an eighth aspect of the present invention, in a liquid drop spraying apparatus comprising a liquid drop spraying nozzle, a pressurizing chamber for pressurizing liquid sprayed from the nozzle, an introducing hole for supplying liquid to the pressurizing chamber, and a piezoelectric/electrostrictive element for operating the pressurizing chamber to be pressurized, wherein the liquid introducing holes of a plurality of the adjacent liquid drop spraying units are coupled with a common liquid supply path, there is provided a discharge circuit for a liquid drop spraying apparatus for discharging the piezoelectric/electrostrictive element, thereby deforming the wall of the pressurizing chamber, and spraying from the nozzle the liquid supplied to the pressurizing chamber with a pressure generated at the pressurizing chamber, wherein the discharge circuit discharges a current from the piezoelectric/electrostrictive element to which a charge voltage has been applied, and then holds a final discharge voltage for a predetermined time. Charging with two or more different charge constants is then performed sequentially, and the first charge time constant is greater than the second charge time constant. An inductance and a resistor are interposed in series with respect to the piezoelectric/electrostrictive element in at least one charge circuit, the piezoelectric/electrostrictive elements are divided into at least two groups, a circuit for charging and discharging a current is provided at their respective groups, and at least part of the discharge current of one group is used for part of the charge current of the other group.
With this arrangement, when liquid drops are sprayed during piezoelectric/electrostrictive element discharging, when charging, the charge time constant of the first charge circuit is large. Thus, the piezoelectric/electrostrictive element can start shape restoration gently. In the case where liquid drops are sprayed from a plurality of liquid drop spraying units simultaneously, an operation for introducing liquid to a plurality of pressure chambers can be reliably performed. Then, this operation goes to a suction operation in which a time required to charge a unit voltage value is short. Thus, liquid can be supplied to the pressure chamber smoothly and within a short time, and the liquid supply quantity can be increased.
In addition, the discharge power of one piezoelectric/electrostrictive element is directly employed as a charge current for the other piezoelectric/electrostrictive element. Thus, there is no need to newly provide discharging means, and further, power consumption can be saved.
According to a ninth aspect of the present invention, as in the seventh aspect thereof, in addition to a discharge circuit, an inductance and a resistor are interposed in series with respect to a charge circuit.
By doing this, the voltage/time gradient during spraying is made linear, and the stability of liquid drops spray is improved.
According to a tenth aspect of the present invention, as in the eighth aspect thereof, in addition to a charge circuit, an inductance and a resistor are interposed in series with respect to a discharge circuit.
By doing this, the voltage/time gradient during spraying is made linear, and the stability of liquid drops spray is improved.